Abstract: A method for producing gradient gel medium membrane for electrophoresis for determining the base sequence of DNA or DNA partially decomposed material providing an improved productivity. High and low concentration monomer solutions are mixed with a predetermined quantity of polymerizing reaction initiator solution by a static mixer to prepare a gel forming solution for coating on a continuously moving web. The flow-rate ratio of the high and low concentration monomer solutions is gradually changed so as to vary the concentration of the monomer in the gel forming solution alternatively from low to high and from high to low along the web.